NEVER! MUUAAHHHAHAHA! sorry about the diabolical laugh, but it really does take a LONG time for ALL the yeast to settle out of solution. I've heard people say that you can cold crash 2 or even 3 times, and still have enough yeast in solution to carbonate in the bottle. I can't speak to the validity of this since I've never cold crashed, but I'd be tempted to say that there will always be yeast in solution...forever and always.